Depends whether you're getting the least expensive model or one of the overclocked options - in terms of performance, it all depends on what you are intending to do with your system. The 960 is a slightly more expensive option but it doubles the CUDA cores and greatly increases your clock speed and ram speed, but it uses more power and puts off more heat so in a small form factor case that might be worth nothing as it could require creative fan placement to keep the rest of your components from feeling the burn while you game. If you're wanting to do a single monitor setup @ 1080p, either card can handle most of the games out there though you might have to decrease from high to medium settings on some of the higher end games to maintain decent FPS, the 960 would obviously have a little more muscle to handle that. If you're planning on setting up Nvidia Surround across multiple monitors, neither of these cards are going to handle that well. I have tested 1080p across 3 displays on my 4GB 770 overclocked card and it wasn't exactly thrilled about the experience so bear that in mind if that is something you were wanting to try.
If I were looking at a new card and didn't need to buy immediately, I would probably wait to be able to increase my budget to get a 970 - additional increase in cuda cores over the 960 and you double your video memory which is cherry for games with additional texture effects like the Battlefield series and Skyrim.